LCDs are becoming more widely used for the display of television type information. Emerging techniques have made a real time display using LCDs a reality. There are numerous manufacturers of LCDs among which 100 by 100, 200 by 200, 300 by 300, etc. LCD element arrays are routinely marketed in the marketplace by such manufacturers as Hughes and Westinghouse. When the nematic crystals employed are driven in the dynamic scattering mode so that when a cell or element of a LCD is energized to have maximum dynamic scattering, it will reflect the incident light and will appear white to a viewer. One type of Hughes' LCDs are driven in the dynamic scattering mode when approximately a negative 20 volt potential difference with respect to their substrates is applied.
Normally, LCD's are addressed one line at a time. For discussion purposes, a 100 by 100 LCD is assumed. Thus the 100 LCD cells forming a line can be accessed simultaneously. In contrast, a standard TV system is scanned sequentially at only one spot at a time.
Since a conventional 525 line television system scans a line in approximately 63.5.mu.s, the input line containing 100 column values should be stationary for that period. However, because the standard TV format video is transmitted at a 5 mHz rate, some sort of a control unit is needed to convert these signals into appropriate amplitudes for actuating the LCD elements. Simultaneously, a synchronizing line addressing pulse of minus 28 volt amplitude is applied to the address line for the duration of each of the line times so that the dynamic scattering of the individual LCD elements is assured.
The foregoing method of operating an LCD is disclosed in the U.S. Pat. No. 3,862,360. The method and apparatus disclosed in the cited patent is a noteworthy advance in the state-of-the-art and it applies analog techniques to implement dual serial to parallel converters, each of which consists of a set of sample and hold, latch and shift register circuits. The method and apparatus call for the use of 100 such converters for the 100 input columns of the LCD. The shift registers are to provide the sequentially enabling pulses for the sampling operation of the incoming video signal. Each sampled signal is further stored by a latch circuit for the length of a line of TV information. During the storage duration, another converter takes on the job of accepting more incoming data.
However meritorious the method and the system of the patented apparatus are, certain areas for improvement are noted. These can be identified mainly as being in the area of frequency limitation and of the large number of sample and hold circuits. Sample and hold circuitry basically consists of field effect transistor switches and capacitors. Fast sampling requires small capacitor values. Yet, a relatively long storage or hold time demands large values of capacitance. Thus, the relatively high 5 mHz sampling rate and the relatively long 63.5.mu.s storage time are contradictory in terms. Necessarily, there must be a compromise. One alternative is to provide for an arrangement of two sets of sample and hold circuits to satisfy the conflicting requirements of speed versus storage times. A consequence of providing two sets is added cost and, thus, the appeal of this approach is somewhat lost. A further limitation of this approach is that it does not lend itself to interface with digital machines such as computers, microprocessors, etc. with a random access capability. Furthermore, it is generally agreed that it is more difficult to implement analog circuits in the long storage interval form.
Thus, there is a continuous need in the state-of-the-art for a digital electronic system to control the operation of an LCD unit and display the pictorial information coming from either a camera or a computer. These displays also should have the capability of being varied in rate, size, and position as well as being able to be expanded on LCD arrays of various sizes and to be operated on a sequential or random access basis.